The present invention relates to a novel liquid composition suitable for forming a highly heat-resistant coating film of an organopolysiloxane and a method for the preparation thereof.
Needless to say, heat resistance is one of the most important requirements for various kinds of synthetic resins used in a variety of applications. In this connection, numbers of heat-resistant synthetic resins have been developed including, for example, polyphenylenes, polyimides, polyquinoxalines, polyisoindroquinazolinediones, polytriazines, fluorocarbon resins, silicone resins, chelate resins and the like.
Typically, most strongest demand for such a heat-resistant polymeric material is increasingly laid by the electronics industry or, in particular, the material industry for electronics technology, in which the heat-resistant polymeric material is desired to withstand a temperature as high as 400.degree. C. or higher in addition to good electric insulation as a matter of course for an electric material. Nevertheless, most of the above named conventional heat-resistant polymeric materials are utilizable practically at a temperature not exceeding 300.degree. C. at the highest. In addition, no good solvents are available for preparing a solution of these polymers so that liquid compositions can hardly be obtained of these heat-resistant polymers suitable for film coating.
Limiting the subject matter to the silicone resins, i.e. organopolysiloxane resins, there have been proposed so-called organopolysiloxane ladder polymers composed of the trifunctional organosiloxane units only and expressed by the formula (RSiO.sub.1.5).sub.n, in which R is a monovalent hydrocarbon group and n is a positive integer corresponding to the number of the siloxane units in a molecule. These ladder polymers are deemed to be the most promising among silicone resins as a material to be used in electronics in respect of the outstanding heat resistance and electric properties although these ladder polymers have a fatal defect as a material used in electronics due to the difficulty in completely removing the impurities of alkali metal and chlorine ions unavoidably entering the polymer in the course of preparation. In addition, the ladder polymers have several problems in respect of the relatively low solubility in organic solvents, low storability, poor spreadability on the substrate surface when a coating film is to be formed thereon, cracking of the coating film by backing at an elevated temperature and so on so that they are never quite satisfactory as a material for forming coating films. Of course, properties of the ladder polymers can widely be modified by selecting the hydrocarbon groups denoted by R in the above formula, which can be a group with bulkiness such as an aryl group, e.g. phenyl, tolyl and naphthyl groups, as well as an isobutyl and isoamyl groups or a group with compactness such as a methyl group. However, the ladder polymers with those bulky hydrocarbon groups have disadvantageously low solubility in a solvent while the ladder polymers having methyl groups are poorly storable in addition to the little improved solubility in solvents so that no liquid coating compositions can be prepared thereof with difficulties in handling.
Accordingly, no liquid coating compositions have yet been developed of an organopolysiloxane as the base ingredient despite the eager demand for such a composition capable of giving a coating film having satisfactorily high heat resistance along with excellent electric properties suitable as a material to be used in the electronics technology.